Within a nuclear reactor, the upper boundary of the reactor core is defined by the upper core plate. The upper ends of the nuclear reactor core fuel assemblies are detachably mounted to the undersurface of the upper core plates. The core contains fuel assemblies including fuel rods within which nuclear fuel pellets are disposed. Each fuel assembly has a plurality of tubes which receive nuclear reactor control rods for controlling the power output of the fuel assemblies and the reactor core. Movement of the nuclear reactor control rods is accomplished by control rod drive mechanisms through control rod drive shafts that extend through the pressure vessel.
The nuclear reactor upper support plate is vertically spaced above the upper core plate. An upper plenum chamber is defined between the upper support plate and the upper core plate. Reactor core coolant in the form of water is conducted through the upper plenum chamber for subsequent flow through the reactor core coolant loop and heat exchange system which is external of the pressure vessel and core barrel. The nuclear reactor control rods may be disposed within the upper plenum chamber when they are withdrawn vertically upwardly out of the core; when the control rods are lowered into the core their respective drive shafts are disposed within the upper plenum chamber. Protection and guidance for the control rods and their drive rods within the upper plenum chamber is provided with respect to the cross-currents of the flowing nuclear reactor core coolant by guide tubes. The guide tubes are interposed between, and connected to, the upper surface of the upper core plate and the upper support plate. In an alternate design, a locking device crimps the nut to the shank of the mounting pin.
Annular flanges are provided at the lower ends of the guide tubes to secure the guide tubes to the upper core plate. Guide tube mounting pins position the guide tube flanges with respect to the upper core plate. The vertically disposed guide tube mounting pins have lower portions which are frictionally engaged within suitable bores defined within the upper core plate. The upper portion of each guide tube mounting pin is engaged with an internal hexagonal nut. Counterbored portions of the guide tube flange are engaged between a shoulder portion of the shank and the mated nut of the pin. A locking device such as a cap, dowel pin and the like are employed to prevent retrograde rotation of the nut relative to the mounting pin wherein the nut could become disengaged from the upper portion of the pin.
Since the inception of the rod cluster control assembly guide tube concept by Westinghouse Electric Corp. in the mid-1960s, the design objective has been to align the lower end of the guide tube to the upper core plate via two resilient guide tube mounting pins commonly referred to as "split pins". These split pins are attached to the guide tube lower flange and are engaged within circular holes in the upper core plate. Also, these split pins have leaves that compress as they enter the upper core plate holes and provide a spring compression load to give the lower end of the guide a degree of end fixity. This permits removal of the guide tube in the event of damage or excessive wear simply by unbolting the upper end where it is attached to the top support plate of the reactor internals and extracting it with a pull force sufficient to overcome the friction generated by the split pin leaf compression. See, e.g., U.S. Pat. No. 4,770,846 to Land et al. and U.S. Pat. No. 4,937,039 to Balog et al., and U.S. Pat. No. 5,035,852 to Land et al., all of which are incorporated by this reference for their description of the structure, installation and removal of split pins in standard holes of upper core plates in pressure vessels of pressurized water reactors.
The material originally chosen for the split pins was Inconel Alloy X-750 because of its high strength and superior mechanical properties and its good wear properties. The greater strength permits higher specified compressive loads in the leaves to achieve a higher degree of rigidity in the pinned end of the guide tube. In use, two split pins are provided in the lower flange of the guide tube and spaced 180 degrees apart to support the guide tube against the steady state loads and postulated accident loads and vibratory forces which could act on the guide tube during normal plant operation, as well as to resist upset or abnormal loads applied to the tube which could occur during postulated pipe break accidents or earthquake conditions. The axes of the two pins are opposed in direction so that each pin provides better restraint in a unique 90 degree opposed direction.
In the late 1970s, stress corrosion was observed in the Inconel Alloy X-750 pins of several nuclear plants. Significant time, money, and effort was spent arriving at a solution to the problem, which involved a revised solution heat treatment of the material and a reduced installation stress of the pin. By early 1988, approximately 60 nuclear facilities had the split pins removed and replaced with new Inconel X-750 split pins having advanced manufacturing and heat treating processes considered sufficient to produce pin longevity. More recent industry events have indicated that the replacement Inconel Alloy X-750 pins with the new design may also be susceptible to stress corrosion.
Clearly, there is a need for an improved guide tube mounting pin that is capable of bearing today's postulated loads (as an Inconel Alloy X-750 pin does) but which is not susceptible to stress corrosion. Such a pin should be rapidly and easily installable within a reactor core without major replacements to the upper internals and with minimum (and most desirably, no) machining operations so as to minimize both the cost of installation and the radiation exposure of the workers. Finally, it would be desirable if the pin were made from relatively inexpensive and easily fabricated material.
In the late 1980s, it was proposed to use stainless steel mounting pins to mount control rod guide tubes to upper core support plates having standard sized holes for receiving the mounting pins. U.S. Pat. No. 4,937,039 discloses that two pin mounting systems using 316 stainless steel replacement pins could be used with guide tubes remote from outlet nozzles but not for guide tubes adjacent to the outlet nozzles because two stainless steel pins were insufficient to satisfy upset or abnormal conditions. Thus, this patent discloses a four pin system using 316 stainless steel replacement mounting pins to support the guide tubes adjacent to the outlet nozzles. Undesirably, however, four pin systems require that two additional holes be drilled in the flanges and upper core support plates for each control rod guide tube adjacent the outlet nozzles in backfitting a reactor vessel. In the early 1990s, a few commercial nuclear reactors were redesigned to include new internals having two pin mounting systems, including enlarged holes (compared with standard sized holes) in the upper core support plates for receiving enlarged 316 stainless steel mounting pins for all of the guide tubes in the reactor vessels.